Combining SCR with PNA for low temperature emission control

Abstract

A catalyst article comprises an SCR catalyst and a NOx adsorber catalyst, where each of these catalysts comprise a metal molecular sieve, each with a different metal. The catalyst article can be close coupled with other components to give a NOX performance advantage from cold start to a combined DOC and SCRF system. Higher NOX conversion is also shown in under-floor location due to NOx storage before SCR light off and selective NH3 slip control, allowing higher NH3 fill levels. Systems comprising the catalyst article and methods of using the catalyst article to give improved hydrocarbon and carbon monoxide control, as well as ammonia slip control, are described. The systems can include flow-through or wall-flow monoliths.

What is claimed is: 
     
       1. A catalyst article comprising an SCR catalyst and a NOx adsorber catalyst, wherein the NOx adsorber catalyst comprises palladium on ceria or a metal-containing molecular sieve, wherein the metal is selected from the group consisting of cerium, chromium, cobalt, copper, iron, lanthanum, manganese, molybdenum, nickel, niobium, palladium, tungsten, silver, vanadium, and zinc, and mixtures thereof. 
     
     
       2. The catalyst article of  claim 1 , wherein the SCR catalyst comprises a base metal, an oxide of a base metal, a molecular sieve, a metal-containing molecular sieve, a metal supported on a mixed oxide or a mixture thereof. 
     
     
       3. The catalyst article of  claim 2 , wherein the base metal is selected from the group consisting of cerium, chromium, cobalt, copper, iron, manganese, molybdenum, nickel, tungsten, and vanadium, and mixtures thereof. 
     
     
       4. The catalyst article of  claim 2 , wherein the SCR catalyst comprises a metal-containing molecular sieve where the metal is selected from the group consisting of cerium, chromium, cobalt, copper, gallium, indium, iridium, iron, manganese, molybdenum, nickel, palladium, platinum, ruthenium, rhenium, silver, tin and zinc. 
     
     
       5. The catalyst article of  claim 4 , wherein the metal in the SCR catalyst comprises copper. 
     
     
       6. The catalyst article of  claim 2 , wherein the metal supported on a mixed oxide comprises at least one catalytic component consisting of (i) at least one transition metal dispersed on a mixed oxide or composite oxide or a mixture thereof as support material consisting of cerium and zirconium; or (ii) cerium oxide and zirconium oxide as single oxides or a composite oxide thereof or a mixture of the single oxides and the composite oxide dispersed on an inert oxide support material, wherein at least one transition metal is dispersed thereon, wherein the at least one transition metal is selected from the group consisting of a Group VIB metal, a Group IB metal, a Group IVA metal, a Group VB metal, a Group VIIB metal, a Group VIII metal and mixtures of any two or more thereof, provided that at least one selected transition metal is tungsten, and the content of cerium and zirconium as oxides in the catalytic component is Ce x Zr 1-X O 2 , wherein X=0.1-0.5. 
     
     
       7. The catalyst article of  claim 2 , wherein the SCR catalyst comprises at least one catalytic component consisting of (i) two or more transition metals dispersed on a mixed oxide or composite oxide or a mixture thereof as support material consisting of cerium and zirconium; or (ii) cerium oxide and zirconium oxide as single oxides or a composite oxide thereof or a mixture of the single oxides and the composite oxide dispersed on an inert oxide support material, whereon is dispersed at least two transition metals, provided that at least one of the metals is tungsten. 
     
     
       8. The catalyst article of  claim 2 , wherein when the molecular sieve in the NOx adsorber catalyst is the same as the molecular sieve in a metal-containing molecular sieve in the SCR catalyst, the metal in the NOx adsorber catalyst and the metal in the SCR catalyst are in combination with the molecular sieve. 
     
     
       9. The catalyst article of  claim 2 , wherein when the molecular sieve in the NOx adsorber catalyst is different than the molecular sieve in a metal-containing molecular sieve in the SCR catalyst, the metal in the NOx adsorber catalyst is in a first combination with the molecular sieve in the NOx adsorber catalyst, the metal in the SCR catalyst is in a second combination with the molecular sieve in the SCR catalyst and the first combination and the second combination are present in a third combination. 
     
     
       10. The catalyst article of  claim 1 , wherein the SCR catalyst comprises an aluminosilicate molecular sieve, an aluminophosphate molecular sieve, a silicoaluminophosphate molecular sieve, a metal-containing aluminosilicate molecular sieve, a metal-containing aluminophosphate molecular sieve, or a metal-containing silicoaluminophosphate molecular sieve. 
     
     
       11. The catalyst article of  claim 10 , wherein the SCR catalyst comprises a molecular sieve or a metal-containing molecular sieve and the molecular sieve or the molecular sieve in the metal-containing molecular sieve comprises erionite or a Framework Type selected from the group consisting of: AEI, BEA (beta zeolites), CHA (Chabazite), FAU (zeolite Y), FER (ferrierite), MFI (ZSM-5) and MOR (mordenite). 
     
     
       12. The catalyst article of  claim 1 , wherein the molecular sieve in the metal-containing molecular sieve in the NOx adsorber catalyst comprises an aluminosilicate, an aluminophosphate, or a silicoaluminophosphate. 
     
     
       13. The catalyst article of  claim 1 , wherein the molecular sieve in the metal-containing molecular sieve comprises an aluminosilicate having an SAR from 10 to 100, inclusive, or a silica aluminophosphate comprising between 5% and 15%, inclusive, of silica. 
     
     
       14. The catalyst article of  claim 1 , wherein the metal in the NOx adsorber catalyst is cobalt, manganese, palladium, or zinc. 
     
     
       15. The catalyst article of  claim 1 , wherein the metal in the NOx adsorber catalyst comprises palladium or zinc. 
     
     
       16. The catalyst article of  claim 1 , wherein the metal in the SCR catalyst is copper and the metal in the NOx adsorber catalyst is palladium. 
     
     
       17. The catalyst article of  claim 1 , wherein (a) when the SCR catalyst comprises a base metal or an oxide thereof, the base metal is present at a concentration of 0.01 to 20 wt. % based on the total weight of the SCR catalyst; or (b) when the SCR catalyst comprises a metal-containing molecular sieve, the metal in the SCR catalyst is present in the molecular sieve at a concentration of 0.01 to 10 wt. %, based on the weight of the molecular sieve. 
     
     
       18. The catalyst article of  claim 1 , wherein the SCR catalyst is present at a loading of about 0.5 to about 4.0 g/in 3 . 
     
     
       19. The catalyst article of  claim 1 , wherein the metal in the NOx adsorber catalyst is present at a concentration of 0.01 to 20 wt. % based on the total weight of the NOx adsorber catalyst. 
     
     
       20. The catalyst article of  claim 1 , wherein the article comprises:
 (a) a first layer comprising the SCR catalyst and a second layer comprising the NOx adsorber catalyst, or 
 (b) a first layer comprising the NOx adsorber catalyst and a second layer comprising the SCR catalyst. 
 
     
     
       21. The catalyst article of  claim 1 , wherein the catalyst article comprises an extruded monolith substrate, wherein:
 (a) the extruded substrate comprises the SCR catalyst and the NOx adsorber catalyst is disposed on the extruded substrate as a layer; or 
 (b) the extruded substrate comprises the NOx adsorber catalyst and the SCR catalyst is disposed on the extruded substrate as a layer; or 
 (c) the extruded substrate comprises the SCR catalyst and the NOx adsorber catalyst. 
 
     
     
       22. An exhaust system comprising a catalyst article of  claim 1 , a passive NOx adsorber (PNA) and optionally a diesel oxidation catalyst (DOC) or catalyzed soot filter (CSF), wherein the passive NOx adsorber or the DOC follow the CSF and are positioned upstream of the catalyst article of  claim 1  and the article of  claim 1  is located on a filter. 
     
     
       23. An exhaust system comprising a catalyst article of  claim 1  and a close coupled catalyzed soot filter (CSF) or an SCR on filter (SCRF), where the catalyzed soot filter is positioned downstream of the catalyst article of  claim 1 . 
     
     
       24. A method for reducing the concentration of NOx in an exhaust gas stream during the cold start of the engine, the method comprising contacting an exhaust gas stream containing NOx with a catalyst article of  claim 1 , wherein stored NOx is reduced by hydrocarbon SCR. 
     
     
       25. A catalytic washcoat comprising an SCR catalyst, a NOx adsorber catalyst and at least one binder, wherein the SCR catalyst comprises a metal selected from the group consisting of cerium, chromium, cobalt, copper, gallium, indium, iridium, iron, manganese, molybdenum, nickel, palladium, platinum, ruthenium, rhenium, silver, tin and zinc; the NOx adsorber catalyst comprises a molecular sieve and a metal selected from the group consisting of cerium, cobalt, iron, lanthanum, manganese, molybdenum, niobium, palladium, tungsten, silver and zinc, wherein:
 (a) the metal in the SCR catalyst and the metal in the NOx adsorber catalyst are different, or 
 (b) the metal in the SCR catalyst and the metal in the NOx adsorber catalyst is the same and either (i) only the NOx adsorber catalyst comprises a molecular sieve, or (ii) the molecular sieve in the SCR catalyst is different than the molecular sieve in the NOx adsorber catalyst, and the at least one binder is selected from the group consisting of alumina, silica, non-zeolite silica-alumina, natural clay, TiO 2 , ZrO 2  and SnO 2 .